This invention relates to flue gas desulfurization processes and more particularly to a process of regenerating sulfur dioxide absorbed in buffered solutions used to scrub sulfur dioxide from gases such as flue gas.
Presently available flue gas desulfurization processes include so-called throw-away systems, utilizing limestone or lime scrubbing, as well as regenerative processes, utilizing buffered aqueous solutions for sulfur dioxide absorption, that yield sulfur as a byproduct. One such regenerative process, using a buffered citrate solution to scrub sulfur dioxide from a gas in a countercurrent absorber and reacting the sulfur dioxide-laden liquor from the absorber with hydrogen sulfide to recover sulfur, is known by the name, CITREX, a trade name of Peabody Engineered Systems, Stamford, Connective. A description of the CITREX process and a discussion of its advantages over throw-away methods such as limestone scrubbing appears in "The CITREX Process for SO.sub.2 Removal", Chemical Engineering Progress, Vol. 71, No. 5, May 1975.
A limestone process of flue gas desulfurization has the drawbacks of sludge disposal and high raw material cost of lime or limestone. By contrast, the citrate process, especially as modified in the CITREX process, provides advantages such as sulfur recovery as a byproduct as well as lower initial and operating costs. Nevertheless, the use of hydrogen sulfide to convert the sulfur dioxide absorbed in the citrate solution to yield sulfur and water may not be commercially attractive in all situations. Thus, where it is necessary to synthesize hydrogen sulfide at a plant site from hydrogen produced from natural gas, the increasing unavailability of natural gas or similar raw material for the production of hydrogen makes such a process less desirable.
In U.S. Pat. No. 1,589,133 to Eustis there is disclosed a method of recovering sulfur dioxide from smaller smoke or other gases by absorbing the sulfur dioxide in a solution of a metallic salt, such as aluminum sulfite, which will form a relatively unstable sulfite or bisulfite with the sulfur dioxide and which will readily liberate the sulfur dioxide gas at moderate temperatures. The sulfur dioxide gas is extracted from the solution by diluting the atmosphere in contact with the solution and consequently reducing the partial pressure of the sulfur dioxide in the gases or atmosphere contacting with the solution. The patentee states that this is done in an extractor into the bottom of which is directly fed live steam and into the top of which is fed the sulfur dioxide containing solution. By making the extractor very large and prolonging solution dwell time a large percentage of the total sulfur dioxide is said to be extracted at each cycle. The extraction may be carried out under pressures below atmospheric, using a vacuum pump, although a vacuum is not necessary as dilution of the atmosphere resulting in reduction of the partial pressure of the sulfur dioxide is said to work satisfactorily when the extraction is carried on at, or even above, atmospheric pressure.
The use of live steam, as in Eustis, is attendant with several disadvantages. The steam used must be produced from water which has been treated to avoid contaminating the stripping system or the constituents therein, resulting in added expense. Also, the live steam condenses in the system and dilutes the solution absorbing the sulfur dioxide so that either further separation or waste discharge is required. The former is uneconomical while the latter is impractical under current environmental procedures as well as a costly use of raw material.